Same situation, missing two tiny slices on the Brute Slicers. Moved on from Siren Alley where I know they randomly spawn, since they guide say they are present in all levels after that. Haven't seen a single Brute since Siren Alley, and I'm about to move into Persephone =/

Here's to hoping that Elite Brute is a special encounter.

Update: Indeed, when you get to the second to last level, Outer Persephone, the Brutes return. Encountered two of them (regular brutes, difficulty related?) at the beginning and unlocked Master Researcher. Note that on Easy, there were zero Brutes after Siren Alley and up until Outer Persephone.